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Full text of "The manufacture and properties of iron and steel"

THE BLAST FURNACE.                                        59
that there is almost no carbon deposition with magnetite, a fact which I have verified by experiment, and it is generally agreed that carbon deposition is essential to good reduction and fuel economy. Nevertheless, Cuban ore has been smelted at Steelton with less than a ton of coke per ton of iron and in a furnace only 65 ft. high, the practice being continued for a long time. This ore is mostly magnetite, in hard lumps, containing 10 per cent, silica, and from 0.25 to 0.50 per cent, sulphur, and on account of this latter impurity it was essential to maintain a good temperature, but this was done so successfully that the iron ran from a trace to- .04 per cent, in sulphur.
It is possible that the volatilization of the sulphur in the upper part of the furnace may make the ore porous, but this explanation does not account for the easy reduction, because the sulphur is not distributed regularly throughout the ore, but is in separate crystals and masses, and under these conditions a content of less than half of one per cent, of sulphur is not enough to produce any great change in porosity. Moreover, this sulphur will not be completely distilled or acted upon in the upper zones of the furnace, where the relative reducibility is-of great consequence. But even assuming that the volatilization of the sulphur renders the ore reducible, this merely proves that magnetite is not as hard to reduce as is generally supposed. It may be that an unusually hard ore like the Swedish magnetites will be less easily reduced than a porous mineral, but it is not logical to say that magnetic oxide is hard to reduce, simply because magnetic oxide usually occurs in hard and compact formations. The correct expression would be that compact ores are hard to reduce and that magnetites axe usually of this character. Even this conclusion is open to dispute, for the Cuban ore above referred to is solid and in lumps, and yet gives as good a fuel ratio as would be expected from its silica content. Moreover, the Swedish magnetites themselves have been used in large quantities in Germany, and it is the experience in more than one works, that no increase in fuel follows their use. I have been given the figures of two furnaces using about 40 per cent, of these ores, where the fuel for a whole campaign ran 1.05 tons of coke per ton of iron, although the burden carried only 42 per cent, of iron, and was in no measure self-fluxing. A large proportion of the charge was puddle cinder, which is not easy to reduce,